This invention relates to a ducted gas turbine engine. More particularly this invention is concerned with the preservation of a gas turbine engine after damage to its fan.
Ducted fan gas turbine engines conventionally comprise a core engine which drives a comparatively large diameter propulsive fan positioned at the upstream end of the core engine. By the nature of the position of the fan at the front of the engine it is most vulnerable to damage by ingestion of debris. Such ingestion of foreign bodies can cause a fan blade to partly or fully break off thus leaving the fan operating out of balance. This usually necessitates the shutting down of the engine to minimise the damage. However the imbalance in the fan created by the part or full blade loss generates extremely high radial loads which must be at least partially absorbed as the engine is run down to windmilling speed. Windmilling speed is the speed at which the engine rotates in a non-operative condition as a result of its motion through atmosphere.
Prior proposals for dealing with such imbalance are disclosed in prior patents GB 2079402 and GB 2130340. Both these patents disclose arrangements where load limitation is achieved by the use of xe2x80x98fuse pinsxe2x80x99. Typically the main bearing supporting the fan shaft is connected to the remainder of the engine structure in the region of the fan by a plurality of axially extending fuse pins. The pins fracture in shear upon a major fan imbalance allowing the fan and its shaft to orbit about the engine""s longitudinal axis. This continues as the engine is allowed to run down to windmilling speed.
However the vibration resulting from the fan imbalance that still exists at windmilling speed can still be extremely severe. This is due mainly to speed coincidence with the reduced natural frequency of vibration of the fan because of the lack of radial stiffness of the fan assembly after fusing of the bearing support load path.
A prior proposal for dealing with such fan vibration during windmilling is disclosed in prior patent GB 2326679. This prior patent discloses an arrangement where a first bearing carrying the upstream part of the fan shaft is connected to the fixed engine structure via a frangible support member. A second bearing carrying the upstream part of the intermediate pressure compressor shaft is attached to the fixed structure of the engine casing by a hairpin shaped resilient support member which is also attached to the mounting of the first bearing.
In the event of the fan suffering damage the frangible support member breaks which ensures that most of the engine is protected from major damage arising from out-of-balance forces and that the fan is able to orbit about the engine longitudinal axis. However, additionally the natural frequency of the fan is raised by the hairpin member providing a resilient connection between the fan and hub structure. The forcing frequency of the windmilling fan should thus be outside the resonance range. The resilient hairpin member also absorbs out-of-balance radial loads.
One problem, arising from the aforementioned arrangement described in GB 2326679 is that the second bearing supporting the intermediate pressure compressor shaft is itself supported by the flexible hairpin connection between the bearings and the casing structure. During normal running the load path provided by the elastic structure between the IP bearing housing and the hub structure is undesirably long and too flexible to provide good support for the IP compressor shaft.
There is a need therefore for a flexible structure which not only provides the advantages disclosed in GB 2326679 but also provides good support for the IP compressor shaft.
A need has also been identified for an arrangement which accommodates the large deflections during the initial stages of a fan-blade-off incident but would also be able to offer stiff support at the smaller deflections during windmilling.
According to the present invention there is provided a ducted fan gas turbine engine having a longitudinal axis and including a propulsive fan mounted on a first shaft and a compressor mounted on a second shaft, said shafts being normally coaxial both with each other and with said engine longitudinal axis, and respectively supported at their upstream ends by axially spaced apart first and second bearing members, said first bearing member being supported from the fixed structure of said engine by support means and being interconnected with said second bearing member by an axial sleeve wherein said support means comprises an axially flexible and radially stiff frustoconical member interconnecting said engine casing and said second bearing member.
Preferably said frustoconical member is positioned at the downstream end of said axial sleeve.
Preferably said flexible frustoconical member is hollow.